Motor vehicle suspension systems are configured so that the wheels are able to follow elevational changes in the road surface as the vehicle travels therealong. When a rise in the road surface is encountered, the suspension responds in “jounce” in which the wheel is able to move upwardly relative to the frame of the vehicle. On the other hand, when a dip in the road surface is encountered, the suspension responds in “rebound” in which the wheel is able to move downwardly relative to the frame of the vehicle. In either jounce or rebound, a spring (i.e., coil, leaf, torsion, etc.) is incorporated at the wheel in order to provide a resilient response to the respective vertical movements with regard to the vehicle frame. However, in order to prevent wheel bouncing and excessive vehicle body motion, a damper (i.e., shock absorber, strut, etc.) is placed at the wheel to dampen wheel bounce. Additionally, when the limit of jounce is encountered, it is customary to provide a maximum jounce impact absorber in the form of a bumper cushion.
Referring now to FIGS. 1 through 1B, components of a conventional suspension system 10 are depicted which allow for jounce and rebound at a wheel of the subject motor vehicle 12.
Firstly with regard to FIG. 1, a control arm 14 is pivotally mounted with respect to the frame 16, wherein, in the depicted example, a torsion spring 18 is utilized to provide resilient response for the jounce and rebound of the control arm relative to the frame. To provide control over the rate of jounce and rebound, a damper in the form of a shock absorber 20 is connected pivotally at one end to the frame 16 and connected pivotally at the other end to the control arm 14. Alternatively, a damper in the form of a strut may be used in the suspension system, as for example disclosed in U.S. Pat. No. 5,467,971. To provide cushioning in the event a maximum jounce occurs, a jounce bumper cushion 22 is mounted to the frame 16 which is resiliently compressed by movement of the control arm as jounce approaches its maximum.
Referring next to FIG. 1A, the internal components and operational aspects of a conventional shock absorber 20′ (a remote reservoir high pressure gas type shock absorber being shown merely by way of example) can be understood. A valved piston 30 is reciprocably movable within a shock cylinder 32. A shock rod 34 is attached to the valved piston 30 and is guided by a shock rod guide 36 at one end of the shock cylinder 32. Below the valved piston 30 and above the shock rod guide 36 is a mutually interacting rebound limiter 38. The instantaneous position of the valved piston 30 within the shock cylinder 32 defines a first interior portion 32F and a second interior portion 32S of the interior of the shock cylinder. In the example depicted at FIG. 1A, the pressurization in the first and second interior portions 32F, 32S is provided by an hydraulic fluid O which is pressurized by pressurized gas, preferably nitrogen, G acting on a divider piston 40 of an hydraulic fluid reservoir cylinder 42, wherein a tube 44, including a base valve 44V, connects the hydraulic fluid between the hydraulic fluid reservoir cylinder and the first interior portion. In operation, as the control arm undergoes jounce, the hydraulic fluid is displaced from the first interior portion into the hydraulic fluid reservoir cylinder, causing the pressure of the nitrogen gas to increase as its volume decreases and thereby causing an increased hydraulic pressure on the valved piston 30 in a direction toward the shock rod guide. Hydraulic fluid is able to directionally meter through valving 46 of the valved piston 30 in a manner which provides damping.
Referring next to FIG. 1B, the internal structure of a conventional jounce bumper cushion 22 can be understood. An optional skin 50 of a compliant material (i.e., having energy absorbing or damping properties) may, or may not, overlay an interior of resilient elastomeric material 52, which may be for example a rubber, rubber-like material, or micro-cellular urethane. In operation as the control arm approaches maximum jounce, the jounce bumper cushion 22 compresses, delivering a reaction force on the control arm which increases with increasing compression so as to minimize the severity of impact of the control arm with respect to the frame at the limit of jounce. Immediately following the jounce, the rebound involves the energy absorbed by the compression of the conventional bumper cushion being delivered resiliently back to the suspension.
In the art of motor vehicle suspension systems, it is known that a conventional jounce bumper cushion and related dampers can show wear. It is also known that when the energy absorbed from a particular bump or dip exceeds the capacity of a conventional jounce bumper cushion, a hard mechanical stop is engaged. This abrupt transfer of jounce force and energy to the frame manifests itself in the passenger compartment as a sharp jolt, which can create load management issues in addition to the discomfort of a rough ride. Further, in order for the frame to accept such impact loads, the structure of the frame must be engineered for an appropriate strength, which is undesirable from the standpoint of the added vehicle weight such structures must inherently entail.
Vehicle suspension engineering has traditionally focused on ride and handling as this pertains to body and wheel relative motion with respect to the body below about 1.5 m/s (meters per second). However, the suspension travel requirements in a vehicle are mainly driven by severe events which generate maximum displacements of the wheel relative to the body. These severe events, such as when the vehicle encounters a deep and steep-walled pothole, can generate wheel velocities (relative to the body) of up to 9 m/s.
An approach pursued by Bavarian Motor Works (BMW) of Munich, Germany, is described in European Patent Application EP 1,569,810 B1, published on Sep. 7, 2005; which application is parent to U.S. Patent Application publication 2006/0243548 A1, published on Nov. 2, 2006.
The object of the BMW disclosure of EP 1,569,810 B1 is to provide a vibration damping method on a motor vehicle wheel suspension by means of a hydraulic vibration damper which prevents great loads on the vehicle body and chassis caused by very large vertical velocities of the wheel, e.g., when traveling over potholes. According to the BMW disclosure, in a hydraulic vibration damper for a motor vehicle, a method of vibration damping on a wheel suspension is used by BMW, characterized in that the damping force of the vibration damper increases as a function of piston speed, especially in the piston speed range of essentially 0 to 2 m/s, at first increasing slowly, essentially linearly, and then, especially above a piston speed of essentially 2 m/s, increasing according to a highly progressive function. Further according to the BMW disclosure, through a suitable choice, design and construction of vibration damper valves or by otherwise influencing the hydraulic resistances in the vibration damper, it is possible to implement a characteristic which is generated by damping forces known from the state of the art in the piston speed range up to the end of the range that is relevant for comfort, and beyond this piston speed range, an extreme progression in the damper characteristic is induced to decelerate the accelerated masses to a greater extent.
While the BMW disclosure seeks to provide a solution to the long-standing problem of damping excessively large wheel-to-body velocities while attempting to maintain acceptable ride and handling for low velocities, the disclosure requires an ad hoc reliance upon a presupposed and essential damper curve which is devoid of any underlying physics which supports any of the curve aspects. Thus, what yet remains needed in the art is an analytical methodology to predict damping curves which truly achieve the goal of damping excessively large wheel-to-body velocities while attempting to maintain acceptable ride and handling for low velocities.
Of additional note is Japan Society of Automotive Engineers, JSAE technical paper 9306714 by Miyazaki, Kiyoaki, Yasai, Hirofumi, “A study of ride improvement of the bus”, JSAE Autumn Convention Nagoya, Japan Oct. 19-21, 1993, wherein the authors confirmed that a progressive damping characteristic is effective for reducing the pitching and impact vibration.
Of further note is Society of Automotive Engineers, SAE technical paper 2006-01-1984 by Benoit Lacroix, Patrice Seers and Zhaoheng Liu, “A Passive Nonlinear Damping Design for a Road Race Car Application”, wherein a nonlinear passive damping design is proposed to optimize the handling performance of an SAE Formula car in terms of roll and pitch responses.
Progressive damping is thought of as an enabler to reduce harsh impact, ride input feel when encountering severe events through the method of maintaining a predefined load in jounce and reducing engagement into the jounce suspension stop. It is also needed to develop enablers to reduce total jounce travel so that a given vehicle could be trimmed lower to enable competitive styling cues. Trimming a vehicle lower usually increases the level of harshness for an event such as a deep pothole and other severe events.
What remains needed in the art, therefore, is an analytical methodology for the specification of progressive optimal compression damping that enables the suspension to negotiate severe events with reduced harshness, yet provides very acceptable ride quality and handling during routine events, limits peak loads on the fame structure, reduces wheel travel, and enables lower trim height.